The present invention relates to the exhaust system of an internal combustion engine and, more specifically, to employing low-density, high-temperature resistant, inorganic spheroids to insulate a double-walled exhaust pipe of an automobile or other motorized vehicles.
With the advent of more and more plastic and electronic components on motorized vehicles, particularly automobiles, it is becoming more important to insulate these items from the hot exhaust system. Presently, individual components or specific areas of the car are protected by heat shields or insulation, or are located a sufficient distance from the exhaust system to avoid heat. Heat shields or insulation can be costly when the item to be protected is large, such as a plastic gasoline tank, and it is not always feasible or practical to locate such items away from the exhaust system. A more economical approach is to insulate the source of the heat. In this case, the exhaust pipe which carries and is heated by the exhaust gas.
Insulating the exhaust pipe can also have other advantages. Catalytic converters must reach a certain temperature before they "light off" or begin to oxidize carbon monoxide and hydrocarbons. Insulating the exhaust pipe between the exhaust manifold and the catalytic converter minimizes heat loss and therefore decreases the time for "light off" to occur. This is very important when the car is first started, especially in cold weather, to satisfy the increasingly stringent air quality standards.
A major difficulty has been to insulate the pipe effectively, easily and economically. Ceramic fiber has been used to insulate exhaust pipes. However, since the ceramic fiber by itself is fragile and its heat insulation property is drastically reduced as it picks up moisture, it must be protected. This requires sheet metal shells to be affixed around the fiber insulation which is expensive, increases the number of parts, and can be a source of noise and rattles. In addition, the fiber may present a hazard if its escapes and becomes airborne because of metal shell breakage or corrosion during the life of the vehicle.
Double-walled exhaust pipes have been employed as a means of insulating hot exhaust gas from vehicle components. A double-walled pipe consists of a pipe within a pipe with a small annular air gap between them. Unfortunately, in many cases, it does not reduce the temperature sufficiently, and can be a source of noise, since the outer pipe is not sufficiently constrained. There have been attempts at filling this space between the two pipes with mineral powder as in European Patent Application 0 285 804 A1. Because of the dusting and caking nature of such powder, it is difficult to apply and to apply uniformly. The powder also adds significant weight to the exhaust system and must be carefully sealed against moisture absorption which has a deleterious effect on insulation properties.
It is an object of this invention to produce an economical, light-weight, low-noise, thermally-insulated, double-walled pipe system that overcomes the difficulties of previous exhaust systems. It is a further object of this invention to provide an insulated exhaust system which in the event of a catastrophic failure such as pipe breakage or corrosion would have a minimum impact on the environment.